There are insertion systems that include an image forming apparatus to form images on envelopes as well as on sheets to be inserted into the envelopes (hereinafter “enclosure”) and an insertion device to automatically insert the enclosures into the envelopes. For example, JP-2004-045650-A proposes a printing unit that includes first and second image forming units to form images on enclosures and envelopes, respectively. Specifically, the first image forming unit is disposed beneath an image reading unit and forms images electrophotographically on predetermined recording sheets fed by a sheet feeding unit, thus forming “quotations” as enclosures. The second image forming unit is disposed above a sheet processing unit and includes an enveloper feeder. The second image forming unit forms images on envelopes fed by the envelope feeder.
In this system, enclosures on which the first image forming unit has formed images are automatically inserted into envelopes on which recipients' addresses have been printed by the second image forming unit so that the contents of the envelopes correspond to the recipients.
The above-described method, however, has the following drawback because the image data size can be significantly different between the images formed by the first image forming unit and those formed by the second image forming unit. Typically, the size of images formed on enclosures is significantly greater than the size of images formed on envelopes. In insertion systems, typically the first image forming unit is provided in the image forming apparatus, and the second image forming unit is provided to the sheet processing unit, that is, a post-processing device, which is communicably connected to the image forming apparatus. The printed enclosure is transported from the image forming apparatus to the sheet post-processing device and inserted into the envelope kept open.
In this case, the time required for image formation by the image forming unit is different from that by the second image forming unit in proportional to the differences in data size. Accordingly, the timing at which conveyance of the enclosure is started must be determined considering differences in image formation time as well as differences in the length of conveyance route. Improper conveyance timing can cause defective insertion or jamming of enclosures or envelopes.